cvise: fix unifdef.patch (crashes otherwise)

Without the change `cvise` does not even parse:

    $ cvise --help |& unnix
      File "/<<NIX>>/cvise-2.12.0/bin/.cvise-wrapped", line 96
        programs['unifdef'] = '/<<NIX>>/unifdef-2.12/bin/unifdef'
        ^^^^^^^^
    SyntaxError: expected 'except' or 'finally' block

Happens because the patch applies to the unrelated bit of the source.

While at it added minimal `cvise --help` test.

pkgs/development/tools/misc/cvise/default.nix

@@ -13,6 +13,8 @@
   pebble,
   psutil,
   pytestCheckHook,
+  testers,
+  cvise,
 }:
 
 buildPythonApplication rec {
@@ -78,6 +80,16 @@ buildPythonApplication rec {
     "test_simple_reduction"
   ];
 
+  passthru = {
+    tests = {
+      # basic syntax check
+      help-output = testers.testVersion {
+        package = cvise;
+        command = "cvise --version";
+      };
+    };
+  };
+
   meta = with lib; {
     homepage = "https://github.com/marxin/cvise";
     description = "Super-parallel Python port of C-Reduce";

pkgs/development/tools/misc/cvise/unifdef.patch

@@ -1,6 +1,7 @@
---- a/cvise.py
-+++ b/cvise.py
-@@ -93,4 +93,5 @@ def find_external_programs():
+--- a/cvise/utils/externalprograms.py
++++ b/cvise/utils/externalprograms.py
+@@ -43,5 +43,6 @@ def find_external_programs():
+ 
      # Special case for clang-format
      programs['clang-format'] = '@CLANG_FORMAT_PATH@'
 +    programs['unifdef'] = '@UNIFDEF@'